Flexible metal conduits made from thin sheet metal ribbons which are convoluted and joined at their edges have been constructed in the past. The so-called "interlocked" conduit was a relatively commonplace example of such conduits. Interlocked conduit employed thin metal ribbons wound helically with their convolutions secured together by reversely curved ribbon edges which were interlocked. The interlocked edges permitted limited play between adjacent convolutions so the conduit could be flexed. U.S. Pat. No. 4,197,728, issued Apr. 15, 1980, discloses a conduit constructed in this manner.
Interlocked metal conduits were, and remain, in widespread use and perform in a generally satisfactory way. In some applications however, these conduits have had inadequate flexibility, convolutions have separated when too much bending was experienced, and the interlocked joints themselves have not exhibited adequate hoop strength. These deficiencies have been primarily limited to applications where relatively thin walled fairly large diameter conduits are specified.
Other flexible metal conduit constructions have been proposed in which flexibility is gained primarily by the manner of formation of the conduit convolutions. U.S. Pat. No. 4,486,484 issued Dec. 4, 1984, discloses such a conduit formed from convoluted metal ribbon which is corrugated. In this construction the helical corrugations provide flexibility while the joint between the convolutions is relatively inflexible. Other corrugated convoluted conduits have been proposed in which the convolution joints are tightly crimped but these have drawbacks related to the joint strength, the joint flexibility and/or the ability of the conduit material to form the joint.
The present invention provides a new and improved flexible metal conduit and method of making it wherein the conduit is formed from a thin corrugated convoluted metal ribbon having convolution edge locking structure in which lapped ribbon edge flanges are spirally rolled into a helical locking tube defining a core having a generally circular cross sectional shape. The tube extends along the convolution junctures and provides for a high degree of flexibility, joint integrity and joint strength.